1. Field of the Invention
The present invention relates generally to the field of orthodontic appliances. More specifically, the present invention discloses a method for fabricating orthodontic appliances that include lingual springs.
2. Statement of the Problem
In dentistry, and more particularly in orthodontics, a central step typically accomplished at the beginning of many types of treatment is the taking of impressions of a patient's teeth. From such negative impressions, positive stone models of a patients upper and lower teeth, gums and palate can be reproduced and in this way, stone models duplicate virtually all aspects of a patient's oral anatomy. The availability of such stone models, called study models, provides the attending orthodontist with full information needed to diagnose an individual's malocclusion and devise an indicated treatment plan. From the study models, the full extent and nature of a malocclusion can be quantified and a viable treatment sequence for correcting a malocclusion can be established, which is known as a patient's treatment plan.
After taking a patient's impressions, an orthodontist may ask his staff to pour multiple, duplicate sets of a patient's models, particularly if it is likely that a patient's treatment plan may require appliances that are to be fabricated by an outside orthodontic support laboratory. These are termed “working models”. With multiple sets of models available at the beginning of orthodontic treatment then, the study models can remain available for the orthodontist to use for establishing the patient's treatment plan and the working models may be sent to an orthodontic laboratory along with the doctor's prescription for fabricating a specific appliance for the patient.
Stone models have many inherent attributes that facilitate fabrication of laboratory-produced appliances. For example, laboratory technicians may first grind inter-proximal material from a stone model to fully expose the crowns of the “stone” first molars. With the upper and lower first molars exposed in this manner, laboratory technicians can select and confirm the exact size from a series of molar band or crown sizes that best fit the patient's molar teeth. Those bands or crowns once identified are typically left in a tight fitting, fully seated position on the molars of the working model.
Since the stone models are poured from fine dental stone, once cured they exhibit the properties of a refractory material. High temperature brazing and soldering operations can be performed directly on or in a working model without smoke, distortion or deterioration of the accuracy of the model. A typical laboratory operation would first include the selection and seating of bands or crowns on the molars as described above. Then, a laboratory technician would skillfully size, adapt and contour various hard stainless steel wires in accordance with features of the model. Such wires would be appropriately positioned adjacent to the seated bands. Once the wires are contoured and positioned in this manner, a lab technician can then use a biocompatible silver-based brazing alloy to braze the wires and bands together as a single assembly. In this way, the superstructure of one type of current laboratory-produced orthodontic appliances is fabricated.
In another common and well-known laboratory process used for fabricating appliances, the surfaces of a model are first coated with a release agent. After receiving such a release coating, dental acrylic (e.g., poly methyl-methacrylate monomer and powder) or other commercially available rigid castable plastic dental materials can be cast directly on or within a stone model. Once such a polymeric mass is cured and removed from the model, it retains all of the subtle contour features and overall shape and size of the palate and the teeth present in the model. In this manner, the familiar orthodontic retainer and many other types of functional or orthopedic orthodontic appliances that consist at least in part of plastic components are formed. Further, various auxiliary appliance components such as expansion screws, springs, hooks, and hinges and other metallic orthodontic appliance components can be potted into the dental acrylic as part of the construction of a typical orthodontic appliance. To pot or embed metallic components into the polymeric superstructure of such an appliance, the components are first positioned within the patient's stone model in a manner specified in the attending doctor's prescription. Once positioned, they are temporarily held in place by a structural wax material commonly used in dental laboratories. Then, the laboratory technician will apply a slurry of uncured dental acrylic that surrounds and partially flows over the components as positioned. Once the dental acrylic cures, the various metallic components will be embedded in place within the dental acrylic.
Typically, the fabrication of an orthodontic appliance by an orthodontic laboratory will first involve seating of bands or crowns that fit upon the anatomy of various teeth in the stone model. Then various stainless steel wires will be adapted and brazed in place. This will then be followed by the casting of dental acrylic. Such acrylic appliances may be of a monolithic configuration, or they may be divided into halves, or into right and left halves with a third front section, where all of these sections are intended to move relative to each other in certain expansive ways. Most appliances will include metallic components in combination with dental acrylic but other appliances may be formed from metallic components only, and yet others are formed entirely of dental acrylic. As a rule, metal-only appliances are considered to be “fixed” in that they are permanently affixed to teeth in the mouth and are removable only by the orthodontist. Appliances that are predominantly acrylic are usually classified as “removable” in that such appliances can be removed and reinserted at any time by the patient.
As described, all of the fabrication steps described are accomplished using the patient's working models as a fabrication pattern or fabrication template. The underlying reason for this, and the advantage of using the stone models in this manner, is that once the appliance is completed by the laboratory and shipped to the attending orthodontist, the appliance can be installed directly in the patient's mouth and upon initial insertion, it will exhibit a desired accurate and intimate fit relationship with the teeth, gums and palate of the patient. The fit achieved through this process applies equally to both fixed and removable type appliances.
The ideal fit achieved by an appliance fabricated using this process was first described above as occurring at the beginning of treatment. However, in other cases, the use of this type of appliance may not be indicated until a later phase of treatment. In all cases, fresh impressions must be taken at the time of fabricating such an appliance to produce stone models that reflect the patient's oral realities and treatment progress at that time. Otherwise the finished appliance will not intimately fit in the patient's mouth as required.
Using laboratory-fabricated appliances for orthodontic correction requires that such appliances embody devices that actively generate corrective forces and at the same time, other devices and features that are intended to remain passive and stationary. Normally, such passive features are the larger superficial features of the appliance that serve to provide stability and retention of the appliance within the mouth through direct contact with the gums, palate and some of the teeth. Such passive features thereby serve as a base, anchor, or foundation for the active features of the appliance.
The active features of an orthodontic appliance are those that are capable of generating and imparting desirable corrective orthodontic forces to the teeth in the orthodontic sense, or corrective forces to the malleable bony structures surrounding the oral cavity in an orthopedic sense. It is possible to categorize such active forces that are typically generated by laboratory-produced appliances into two groups. First are those forces that are generated by periodic activation of sub-components of the appliance (e.g., expansion screws). A second category of active forces are those forces that are generated by the loading of various types of springs that store energy.
The latter category of active devices typically employed in laboratory-produced orthodontic appliances that store energy includes various types of springs, which are skillfully positioned within the stone model during fabrication of the appliance. Important to the appreciation of the present inventive methodology, such springs must be positioned anticipatorily. In other words, the laboratory technician must place them within the model in a way that would hypothetically render them passive only at a hypothetical future point in time where the teeth have all been moved to their ideal positions. In yet other words, a laboratory technician must anticipate the ideal corrected positions of each maloccluded tooth, and position any springs associated with that tooth in a positional relationship to that tooth so that after the spring has unloaded its stored energy over time, it has moved the tooth into its corrected position. And in doing so, the spring then becomes passive to that ideally-positioned tooth and will move it no further. To a large extent, the ability of a laboratory technician to achieve this complex and anticipatory task determines his or her level of skill.
Carrying the concept of anticipatorily positioning of springs further, it can be appreciated that upon first placing such a configured appliance in a patient's mouth, each anticipatorily-positioned spring will be deflected by the tooth it is intended to move and thereby loaded according to the degree to which the pre-treatment position of that tooth varies from its ideal position. As can be seen then, it is the action of the mal-positioned teeth acting against, and deflecting the springs that loads the springs, and it is the resulting stored energy from such loading that is slowly dissipated over time as the teeth positionally respond, and over time move in desired directions.
Based on the described need for a laboratory technician to accurately anticipate the teeth in their final and finished aesthetic positions while fabricating an orthodontic appliance, it has become a well known practice for the laboratory technician to undertake a time-consuming and laborious process called “resetting the model.” A reset model is identical to an original model except that it will exhibit most or all of the stone teeth in an ideal position or occlusion, having been cut free of the model and from each other, and then skillfully and individually repositioned into ideal occlusion.
Resetting a stone model in this manner requires the use of special thin saws to cut between each tooth to a point well below the gingival margins of the teeth. In practice, two divergent cuts are actually made between each tooth and its adjacent tooth. The divergent configuration of the two cuts between adjacent teeth creates a tapered wedge of standing stone material that can be broken out free of the model. After that, the individual teeth themselves can be similarly snapped free from the model. The laboratory technician will then reposition the teeth on the model using a heated wax material known as “base plate wax” that hardens at room temperature.
The wedge of stone material described above as being removed from between adjacent teeth creates open space associated with the roots of the teeth that permits the freedom for a tooth to be uprighted in angulation if required. In rotation, the stone teeth can be individually manipulated and positioned by the laboratory technician and aligned by the use of thumb and forefinger pressure applied to a group of teeth and similarly, a single tooth or a group of teeth can be adjusted in torque by finger manipulation.
The wax material bonds the repositioned teeth securely to the model, but the entire model can be heated to soften the wax. Once softened, subtle adjustments can be made to a single tooth or groups of teeth, allowing the technician to ensure that he or she has placed the stone teeth in a close approximation of an ideal occlusion to the degree that such a resetting process permits.
During the process of fabricating an orthodontic appliance and specifically after the step of resetting the model, the dental acrylic components of an appliance can be flowed into and cast within the model. During this process, but prior to actually applying the uncured slurry of acrylic powder and monomer, the laboratory technician can place springs passively in contact with the “corrected” stone teeth. Such spring devices are similarly held in their desired positions by the wax material. It is important to note that once the dental acrylic is applied and cured, the springs will then be held rigidly in their pre-determined positions, with their active end resting passively against the corrected stone teeth, and with stem embedded rigidly in the polymeric substrate.
The incorporation of the resetting step during the fabrication process greatly reduces the challenge and complexity of the task faced by the laboratory technician where he or she must anticipate and imagine the ideal final positions of the teeth. By resetting the teeth into their final, treated positions, all of the guessing is eliminated. Considering all of this then, one can see that it is the true positional discrepancy between the ideally placed stone teeth and the patient's actual mal-occluded teeth that will deflect and thereby load the springs as an appliance fabricated in this manner is initially placed in the mouth and pushed into a fully-seated position at an initial trial fitting.
Even though the step of resetting a patient's stone models is undertaken to reduce the anticipated challenge and complexity of fabricating a quality appliance, and to more closely approximate the finished results of treatment, the resetting process itself introduces a number of problematic shortcomings. First, subtle variances or excesses of inward or outward canting of the reset teeth in torque can drive a significant cumulative error in total arch length. In other words, the sum of effective tooth widths around an arch can vary significantly depending on any subtle lingual or labial inclinations of the teeth. The difference between total arch length of an appliance built from a reset stone model and the patient's actual arches can typically amount to 2 mm or more. In such a case, an appliance may fit poorly in the patient's mouth and may be capable of generating excessive binding forces to the teeth. Such excessive forces can injure the supporting structures of the teeth as well as cause patient discomfort. Another shortcoming of the resetting process is that after the teeth have been repositioned, the relationship between the reset crowns of the teeth and the underlying supportive bone may have been disrupted. For example, if the repositioned crown predicts that the root of the tooth will be located outside of what is known as the alveolar trough, the resulting appliance will be predisposed to move the teeth and their roots into an area of bone that cannot mechanically support the root, or worse, an area of bone where the root cannot be orthopedically moved too. An appliance that is predisposed to attempt to achieve these impossibilities is destined to cause major problems for both the orthodontist and patient alike.
There is also another problem associated with resetting the teeth involving those teeth that may already be in desirable positions and therefore do not require movement. Portions of an acrylic appliance may be cast so that the acrylic is intentionally encouraged to flow up and onto a portion of the dental crown of a tooth. When cured, that portion of the acrylic in intimate contact with the tooth can serve to stabilize and hold the tooth in precise position. Through the resetting process, however, that precise intimate fit relationship between the acrylic and the tooth is lost. The employment of such an accurate fit between some of the teeth and the acrylic portion of an appliance cannot be achieved with an appliance fabricated from a reset model because the necessary positional accuracy is lost during the resetting process.
As can be appreciated from all of the foregoing, the step of resetting a stone model is undertaken in an attempt to increase the accuracy and effectiveness of the appliance that emerges from the stone model and to reduce the complexity and challenge in imagining the finished positions of the teeth. Resetting requires specialized skills on the part of the laboratory technician and a significant amount of time. It involves a great deal of cutting of the stone model and an associated mess, and as described above, the reset model has shortcomings resulting from the resetting process itself. All of these factors combine to increase the cost, complexity and administrative logistics faced by an orthodontist in delivering orthodontic treatment.
3. Solution to the Problem
In contrast to prior art methodologies, the present invention uses a number of ideal reference grooves cut into the lingual surfaces of the model teeth to position lingual springs in an orthodontic appliance. The present methodology serves to achieve all of the advantages of the resetting process while avoiding the various problems, cost and shortcomings associated with the resetting process.